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- Author or Editor: T. Yamagata x
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Abstract
We have investigated the evolution of the Mindanao Dome off the Philippine coast using the GFDL ocean model. It is found that the model's Mindanao Dome evolves in late fall due to local upwelling when a positive curl associated with the northeast Asian winter monsoon increases over the region. It expands eastward with a recirculation composed of the North Equatorial Current in the north, the Mindanao Current in the west, and the North Equatorial Countercurrent in the south. After reaching a maximum in winter, it begins to decay in spring due to an intrusion of downwelling long Rossby waves excited in winter by the northeast trade winds farther eastward near 160°E, as well as a retreat of the local positive wind-stress curl. Further control runs demonstrate that the variation of the model's Mindanao Dome is almost perfectly determined by the change of the wind field in the western Pacific west of the date line. A possible link between the Asian winter monsoon and the seawater temperature anomaly in the western Pacific appears to be the origin of the biennial oscillation of the heat content anomaly in the western tropical Pacific.
This study suggests that the Asian monsoon system, i.e., the air–sea–land system, may strongly regulate oceanic conditions both seasonally and interannually in the tropical Pacific west of the date line.
Abstract
We have investigated the evolution of the Mindanao Dome off the Philippine coast using the GFDL ocean model. It is found that the model's Mindanao Dome evolves in late fall due to local upwelling when a positive curl associated with the northeast Asian winter monsoon increases over the region. It expands eastward with a recirculation composed of the North Equatorial Current in the north, the Mindanao Current in the west, and the North Equatorial Countercurrent in the south. After reaching a maximum in winter, it begins to decay in spring due to an intrusion of downwelling long Rossby waves excited in winter by the northeast trade winds farther eastward near 160°E, as well as a retreat of the local positive wind-stress curl. Further control runs demonstrate that the variation of the model's Mindanao Dome is almost perfectly determined by the change of the wind field in the western Pacific west of the date line. A possible link between the Asian winter monsoon and the seawater temperature anomaly in the western Pacific appears to be the origin of the biennial oscillation of the heat content anomaly in the western tropical Pacific.
This study suggests that the Asian monsoon system, i.e., the air–sea–land system, may strongly regulate oceanic conditions both seasonally and interannually in the tropical Pacific west of the date line.
Abstract
A regional ocean circulation model with fine horizontal resolution has been developed in order to obtain a coherent seasonal picture of the eastern tropical Pacific off Central America.
The Costa Rice Dome with a cyclonic circulation grows rapidly in late spring off the Gulf of Papagayo and matures in summer and early fall in accord with the northward migration of the ITCZ. In winter strong northern converging in the southernmost ITCZ from three passes in Central America excite warm anticyclones nonlinear eddies confined in the upper layer. Those anticyclones are identified as the intermediate geostrophic eddies by Matsuura and Yamagata.
The Costa Rica Dome is eroded in winter and early spring by the westward propagating warm anticyclones but, at the same time, a new embryo of the dome begins to evolve from the westward propagating cold cyclonic eddy excited off the Gulf of Papagayo by the northers. The Costa Rica Dome can be maintained by the winds with the cyclonic wind stress curl in summer only when the embryo of a cyclonic eddy is preconditioned in winter and early spring. In this respect the Costa Rica Dome may be classified into a nonlinear planetary mode, rather than a response to the local wind stress curl.
Abstract
A regional ocean circulation model with fine horizontal resolution has been developed in order to obtain a coherent seasonal picture of the eastern tropical Pacific off Central America.
The Costa Rice Dome with a cyclonic circulation grows rapidly in late spring off the Gulf of Papagayo and matures in summer and early fall in accord with the northward migration of the ITCZ. In winter strong northern converging in the southernmost ITCZ from three passes in Central America excite warm anticyclones nonlinear eddies confined in the upper layer. Those anticyclones are identified as the intermediate geostrophic eddies by Matsuura and Yamagata.
The Costa Rica Dome is eroded in winter and early spring by the westward propagating warm anticyclones but, at the same time, a new embryo of the dome begins to evolve from the westward propagating cold cyclonic eddy excited off the Gulf of Papagayo by the northers. The Costa Rica Dome can be maintained by the winds with the cyclonic wind stress curl in summer only when the embryo of a cyclonic eddy is preconditioned in winter and early spring. In this respect the Costa Rica Dome may be classified into a nonlinear planetary mode, rather than a response to the local wind stress curl.
Abstract
A study of the detailed spatiotemporal characteristics of the Indian Ocean dipole (IOD) mode in SST and surface winds using available observations from 1958 till 1997 is reported. The analysis is used to address several of the controversial issues regarding the IOD.
One key finding of this study is that interdecadal fluctuations contribute strongly to tropical Indian Ocean (TIO) SST variability; in SST anomalies (SSTA) interdecadal variance is as strong as interannual variance. Over both the western and eastern TIO, an accelerated warming of SST after the mid-1970s is apparent. The lack of anticorrelation between western and eastern TIO SSTA occurs only in this latter half of the analysis period.
In order to examine the hypothesis that the IOD is a part of ENSO evolution in the TIO, the temporal characteristics of IOD indices have been compared with Niño-3. On the basis of several quantitative comparisons that include wavelet and cross-wavelet analysis, several important differences between the two phenomena are reported. These differences are highlighted to argue that the IOD is not a part of ENSO evolution in the TIO. On the other hand, a striking similarity is found in the temporal structure of atmospheric and oceanic variability within the TIO that is suggestive of IOD arising from inherent coupled air–sea interactions in the TIO.
ENSO events that do not co-occur with IOD have been isolated and their impacts on TIO SSTA and winds described. Similarly, the characteristics of IOD events that occur independently of ENSO are described. Based on the characteristics of these two groups a hypothesis is suggested through which both phenomena may interact. It is noted that ENSO events co-occurring with IOD events are much stronger compared to non-co-occurring events. On the other hand, IOD events that are independent of ENSO as well as those that co-occur with it appear to have the same strength.
Abstract
A study of the detailed spatiotemporal characteristics of the Indian Ocean dipole (IOD) mode in SST and surface winds using available observations from 1958 till 1997 is reported. The analysis is used to address several of the controversial issues regarding the IOD.
One key finding of this study is that interdecadal fluctuations contribute strongly to tropical Indian Ocean (TIO) SST variability; in SST anomalies (SSTA) interdecadal variance is as strong as interannual variance. Over both the western and eastern TIO, an accelerated warming of SST after the mid-1970s is apparent. The lack of anticorrelation between western and eastern TIO SSTA occurs only in this latter half of the analysis period.
In order to examine the hypothesis that the IOD is a part of ENSO evolution in the TIO, the temporal characteristics of IOD indices have been compared with Niño-3. On the basis of several quantitative comparisons that include wavelet and cross-wavelet analysis, several important differences between the two phenomena are reported. These differences are highlighted to argue that the IOD is not a part of ENSO evolution in the TIO. On the other hand, a striking similarity is found in the temporal structure of atmospheric and oceanic variability within the TIO that is suggestive of IOD arising from inherent coupled air–sea interactions in the TIO.
ENSO events that do not co-occur with IOD have been isolated and their impacts on TIO SSTA and winds described. Similarly, the characteristics of IOD events that occur independently of ENSO are described. Based on the characteristics of these two groups a hypothesis is suggested through which both phenomena may interact. It is noted that ENSO events co-occurring with IOD events are much stronger compared to non-co-occurring events. On the other hand, IOD events that are independent of ENSO as well as those that co-occur with it appear to have the same strength.
Decadal wintertime variability in the North Pacific climate system observed over the last few decades is documented. The decadal sea surface temperature (SST) variability is found to be concentrated around two major oceanic fronts. The variability around the subtropical front, accompanied by the anomalous subtropical high, exhibits strong negative simultaneous correlation with the tropical SST variability, but that around the subarctic front does not. In fact, cooling around the subarctic front in the mid-1970s cannot be attributed to the influence through the atmosphere of tropical warming that occurred about two years later. During the coolest period around the subarctic front in the mid-1980s, the enhanced surface westerlies associated with the intensified Aleutian low seemed to reinforce the underlying SST anomalies. The westerlies tended to be substantially weaker during the warmest period around 1970. These findings are suggestive of self-maintaining mechanisms inherent to the northern North Pacific climate system for the decadal variability.
Decadal wintertime variability in the North Pacific climate system observed over the last few decades is documented. The decadal sea surface temperature (SST) variability is found to be concentrated around two major oceanic fronts. The variability around the subtropical front, accompanied by the anomalous subtropical high, exhibits strong negative simultaneous correlation with the tropical SST variability, but that around the subarctic front does not. In fact, cooling around the subarctic front in the mid-1970s cannot be attributed to the influence through the atmosphere of tropical warming that occurred about two years later. During the coolest period around the subarctic front in the mid-1980s, the enhanced surface westerlies associated with the intensified Aleutian low seemed to reinforce the underlying SST anomalies. The westerlies tended to be substantially weaker during the warmest period around 1970. These findings are suggestive of self-maintaining mechanisms inherent to the northern North Pacific climate system for the decadal variability.
Abstract
Remote effects due to the tropical disturbances in the north Indian Ocean are investigated by analyzing long-lasting (≥5 days) tropical disturbances, which reached at least the strength of tropical storms. The present analysis is carried out for both the pre- and postmonsoon periods. The spatial and temporal distribution of the outgoing longwave radiation (OLR) during the premonsoon disturbances over the Bay of Bengal reveals several interesting features. Temporal distribution of the OLR anomalies shows that the intraseasonal oscillations play an important role in the formation of those disturbances. The spatial distribution of the OLR anomalies shows a dipole with negative OLR anomalies over the bay and positive OLR anomalies over the Indonesian region. The atmospheric response to the negative OLR anomalies results in positive temperature anomalies over northwest India, Pakistan, Afghanistan, Iran, and Saudi Arabia, remote from the disturbance; and the response to the positive anomalies causes slight increase in the sea surface temperature of the Arabian Sea. Negative OLR anomalies are also seen over western Japan due to the Rossby waves generated by the heating over the Bay of Bengal besides the enhancement of the so-called “Pacific–Japan” teleconnection pattern. However, the analysis shows that the postmonsoon disturbances over the Bay of Bengal and the disturbances formed over the Arabian Sea in both pre- and postmonsoon seasons do not develop remote teleconnections associated with the above type of Rossby wave mechanism. These results are significant for the short- to medium-range weather forecast over a wide range covering Japan, Pakistan, Afghanistan, Iran, and Saudi Arabia.
Abstract
Remote effects due to the tropical disturbances in the north Indian Ocean are investigated by analyzing long-lasting (≥5 days) tropical disturbances, which reached at least the strength of tropical storms. The present analysis is carried out for both the pre- and postmonsoon periods. The spatial and temporal distribution of the outgoing longwave radiation (OLR) during the premonsoon disturbances over the Bay of Bengal reveals several interesting features. Temporal distribution of the OLR anomalies shows that the intraseasonal oscillations play an important role in the formation of those disturbances. The spatial distribution of the OLR anomalies shows a dipole with negative OLR anomalies over the bay and positive OLR anomalies over the Indonesian region. The atmospheric response to the negative OLR anomalies results in positive temperature anomalies over northwest India, Pakistan, Afghanistan, Iran, and Saudi Arabia, remote from the disturbance; and the response to the positive anomalies causes slight increase in the sea surface temperature of the Arabian Sea. Negative OLR anomalies are also seen over western Japan due to the Rossby waves generated by the heating over the Bay of Bengal besides the enhancement of the so-called “Pacific–Japan” teleconnection pattern. However, the analysis shows that the postmonsoon disturbances over the Bay of Bengal and the disturbances formed over the Arabian Sea in both pre- and postmonsoon seasons do not develop remote teleconnections associated with the above type of Rossby wave mechanism. These results are significant for the short- to medium-range weather forecast over a wide range covering Japan, Pakistan, Afghanistan, Iran, and Saudi Arabia.
Abstract
A 2.5-layer thermodynamic ocean model is used to investigate interannual variability in sea surface temperature (SST) of the tropical Indian Ocean. Simulated SST agrees well with the data. Model and observed SSTs exhibit large seasonal and interannual variability in the western and southeastern tropical Indian Ocean. Three processes, namely, latent heat flux, radiative flux, and entrainment, play major roles in the evolution of model SST anomalies. Interannual heat flux is found to have greater influence on the SST anomalies in most parts of the model domain. On the other hand, influence of interannual wind is only pronounced near the coasts in the Arabian Sea during the Asian summer monsoon season and a region in the central part of the southern tropical Indian Ocean (STIO) during boreal winter.
Besides the El Niño–Southern Oscillation related basinwide warming, empirical orthogonal function analysis shows a dipole structure in both model and observed SST anomalies in the STIO. The eastern pole is anomalously cold with a peak occurring 4–8 months prior to the negative peak of the Southern Oscillation. A similar dipole structure in the latent heat flux anomalies explains the dipole in the SST anomalies.
Abstract
A 2.5-layer thermodynamic ocean model is used to investigate interannual variability in sea surface temperature (SST) of the tropical Indian Ocean. Simulated SST agrees well with the data. Model and observed SSTs exhibit large seasonal and interannual variability in the western and southeastern tropical Indian Ocean. Three processes, namely, latent heat flux, radiative flux, and entrainment, play major roles in the evolution of model SST anomalies. Interannual heat flux is found to have greater influence on the SST anomalies in most parts of the model domain. On the other hand, influence of interannual wind is only pronounced near the coasts in the Arabian Sea during the Asian summer monsoon season and a region in the central part of the southern tropical Indian Ocean (STIO) during boreal winter.
Besides the El Niño–Southern Oscillation related basinwide warming, empirical orthogonal function analysis shows a dipole structure in both model and observed SST anomalies in the STIO. The eastern pole is anomalously cold with a peak occurring 4–8 months prior to the negative peak of the Southern Oscillation. A similar dipole structure in the latent heat flux anomalies explains the dipole in the SST anomalies.
Abstract
Remote effects modulating the austral summer precipitation over southern Africa during El Niño/El Niño Modoki events are investigated by analyzing the observed events during December–February of the years from 1982/83 to 2010/11. Based on the composite analyses, it is found that southern Africa experiences significantly below normal precipitation during El Niño events compared to El Niño Modoki events. During these latter events, precipitation anomalies are not so significant although southern Africa as a whole receives below normal precipitations. The differences in the spatial distribution of precipitation over southern Africa are seen to be related to the sea surface temperature (SST) anomalies of the equatorial Pacific through atmospheric teleconnections.
The low-level (850 hPa) Matsuno–Gill response to anomalously high precipitation over the Pacific during El Niño events results in an anomalous anticyclone extending from the equatorial to the subtropical South Indian Ocean. These anomalous anticyclonic winds weaken the tropical moisture flow into the southern Africa landmass. Rossby wave activity flux analysis of the upper-level (300 hPa) circulation shows an anomalous tropospheric stationary wave from the Pacific propagating toward southern Africa and maintaining an anomalous anticyclone over southern Africa. The anomalous Matsuno–Gill response and the anomalous tropospheric stationary wave response are intense during El Niño events, causing drought over southern Africa. During El Niño Modoki events, these processes are weaker compared to El Niño events.
Abstract
Remote effects modulating the austral summer precipitation over southern Africa during El Niño/El Niño Modoki events are investigated by analyzing the observed events during December–February of the years from 1982/83 to 2010/11. Based on the composite analyses, it is found that southern Africa experiences significantly below normal precipitation during El Niño events compared to El Niño Modoki events. During these latter events, precipitation anomalies are not so significant although southern Africa as a whole receives below normal precipitations. The differences in the spatial distribution of precipitation over southern Africa are seen to be related to the sea surface temperature (SST) anomalies of the equatorial Pacific through atmospheric teleconnections.
The low-level (850 hPa) Matsuno–Gill response to anomalously high precipitation over the Pacific during El Niño events results in an anomalous anticyclone extending from the equatorial to the subtropical South Indian Ocean. These anomalous anticyclonic winds weaken the tropical moisture flow into the southern Africa landmass. Rossby wave activity flux analysis of the upper-level (300 hPa) circulation shows an anomalous tropospheric stationary wave from the Pacific propagating toward southern Africa and maintaining an anomalous anticyclone over southern Africa. The anomalous Matsuno–Gill response and the anomalous tropospheric stationary wave response are intense during El Niño events, causing drought over southern Africa. During El Niño Modoki events, these processes are weaker compared to El Niño events.
Abstract
During El Niño Southern Oscillation events modest anomalies amplify spatially and temporally until the entire tropical Pacific Ocean and the global atmospheric circulation are affected. Unstable interactions between the ocean and atmosphere could cause this amplification when the release of latent heat by the ocean affects the atmosphere in such a manner that the altered surface winds induce the further release of latent heat. Coupled shallow water models are used to simulate this instability which is modulated by the seasonal movements of the atmospheric convergence zones.
Abstract
During El Niño Southern Oscillation events modest anomalies amplify spatially and temporally until the entire tropical Pacific Ocean and the global atmospheric circulation are affected. Unstable interactions between the ocean and atmosphere could cause this amplification when the release of latent heat by the ocean affects the atmosphere in such a manner that the altered surface winds induce the further release of latent heat. Coupled shallow water models are used to simulate this instability which is modulated by the seasonal movements of the atmospheric convergence zones.
Abstract
The twentieth-century simulations using by 17 coupled ocean–atmosphere general circulation models (CGCMs) submitted to the Intergovernmental Panel on Climate Change’s Fourth Assessment Report (IPCC AR4) are evaluated for their skill in reproducing the observed modes of Indian Ocean (IO) climate variability. Most models successfully capture the IO’s delayed, basinwide warming response a few months after El Niño–Southern Oscillation (ENSO) peaks in the Pacific. ENSO’s oceanic teleconnection into the IO, by coastal waves through the Indonesian archipelago, is poorly simulated in these models, with significant shifts in the turning latitude of radiating Rossby waves. In observations, ENSO forces, by the atmospheric bridge mechanism, strong ocean Rossby waves that induce anomalies of SST, atmospheric convection, and tropical cyclones in a thermocline dome over the southwestern tropical IO. While the southwestern IO thermocline dome is simulated in nearly all of the models, this ocean Rossby wave response to ENSO is present only in a few of the models examined, suggesting difficulties in simulating ENSO’s teleconnection in surface wind.
A majority of the models display an equatorial zonal mode of the Bjerknes feedback with spatial structures and seasonality similar to the Indian Ocean dipole (IOD) in observations. This success appears to be due to their skills in simulating the mean state of the equatorial IO. Corroborating the role of the Bjerknes feedback in the IOD, the thermocline depth, SST, precipitation, and zonal wind are mutually positively correlated in these models, as in observations. The IOD–ENSO correlation during boreal fall ranges from −0.43 to 0.74 in the different models, suggesting that ENSO is one, but not the only, trigger for the IOD.
Abstract
The twentieth-century simulations using by 17 coupled ocean–atmosphere general circulation models (CGCMs) submitted to the Intergovernmental Panel on Climate Change’s Fourth Assessment Report (IPCC AR4) are evaluated for their skill in reproducing the observed modes of Indian Ocean (IO) climate variability. Most models successfully capture the IO’s delayed, basinwide warming response a few months after El Niño–Southern Oscillation (ENSO) peaks in the Pacific. ENSO’s oceanic teleconnection into the IO, by coastal waves through the Indonesian archipelago, is poorly simulated in these models, with significant shifts in the turning latitude of radiating Rossby waves. In observations, ENSO forces, by the atmospheric bridge mechanism, strong ocean Rossby waves that induce anomalies of SST, atmospheric convection, and tropical cyclones in a thermocline dome over the southwestern tropical IO. While the southwestern IO thermocline dome is simulated in nearly all of the models, this ocean Rossby wave response to ENSO is present only in a few of the models examined, suggesting difficulties in simulating ENSO’s teleconnection in surface wind.
A majority of the models display an equatorial zonal mode of the Bjerknes feedback with spatial structures and seasonality similar to the Indian Ocean dipole (IOD) in observations. This success appears to be due to their skills in simulating the mean state of the equatorial IO. Corroborating the role of the Bjerknes feedback in the IOD, the thermocline depth, SST, precipitation, and zonal wind are mutually positively correlated in these models, as in observations. The IOD–ENSO correlation during boreal fall ranges from −0.43 to 0.74 in the different models, suggesting that ENSO is one, but not the only, trigger for the IOD.
Abstract
A modified variational algorithm, previously proposed in meteorology, is presented for the interpolation of oceanic hydrographic and velocity data. The technique is anisotropic and involves a variational approach that allows revealing of the spatial structure in its application. Being a part of the variational family of algorithms, the method is quite general in that it allows one to set dynamical constraints, and weighting functions, applicable to the problem of interest. This flexibility is illustrated by using the nonlinear terms of momentum balance equation as constraints. The inclusion of these constraints appears to assist in the resolution of narrow jets in the flow fields. The method is applied to data from two different regions of the ocean: Lagrangian drifter data from the northwest Pacific and hydrographic data from the Scotian Shelf. Each dataset presents quite different scales, physical processes, and data types. The resulting flow fields are compared with results determined from traditional optimal interpolation, and advantages of the proposed method are discussed.
Abstract
A modified variational algorithm, previously proposed in meteorology, is presented for the interpolation of oceanic hydrographic and velocity data. The technique is anisotropic and involves a variational approach that allows revealing of the spatial structure in its application. Being a part of the variational family of algorithms, the method is quite general in that it allows one to set dynamical constraints, and weighting functions, applicable to the problem of interest. This flexibility is illustrated by using the nonlinear terms of momentum balance equation as constraints. The inclusion of these constraints appears to assist in the resolution of narrow jets in the flow fields. The method is applied to data from two different regions of the ocean: Lagrangian drifter data from the northwest Pacific and hydrographic data from the Scotian Shelf. Each dataset presents quite different scales, physical processes, and data types. The resulting flow fields are compared with results determined from traditional optimal interpolation, and advantages of the proposed method are discussed.
